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CT scan

A CT scan uses X-rays to make a three-dimensional image of a cross-section (slice) of the inside of your body.

CT scans are usually carried out by a radiographer (a health professional trained to perform imaging procedures). It can be used to diagnose and monitor a number of different health conditions, such as suspected appendicitis or certain types of cancer. CT scans can also help to assist with other procedures or treatments. For example, if you’re having a biopsy (a small sample of tissue taken) to diagnose a disease, a CT scan may be used to help guide the needle to the right position.

A CT scanner is a large, ring-shaped machine with a hole in the centre. Inside the ring is an X-ray tube that produces a fan-shaped beam of X-rays. As you lie flat on the scanner table, the tube rotates around you and creates individual images of cross-sections of your body. A computer then joins the individual images together. This will often involve producing several sets of images taken in different directions and also some three-dimensional images. The images from a CT scan appear in different shades of grey.

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  • Preparation Preparing for a CT scan

    CT scans are routinely done as an outpatient procedure. This means you will have the scan and go home the same day.

    Your radiographer will discuss with you what will happen before, during and after your scan. This is your opportunity to understand what will happen. You can help yourself by preparing questions to ask about the risks, benefits and any alternatives to the procedure. This will help you to be informed, so you can give your consent for the procedure to go ahead.

    You must tell your radiographer if you have:

    • asthma
    • diabetes
    • kidney problems
    • any allergies – particularly to contrast medium

    Also, if you have claustrophobia (an extreme or irrational fear of confined places), tell your radiographer before the scan is started.

    You may be asked not to eat or drink anything for a few hours beforehand, particularly if you’re having a CT scan of your abdomen (tummy).

    A dye called contrast medium may be used to make your tissues show up more clearly on the images produced by the scan. Depending on which area of your body needs to be scanned, you may be asked to swallow the contrast medium. Otherwise it may be injected into a vein in your hand or arm, or inserted into your rectum (back passage). If you’re given an injection of contrast medium, this usually gives a warm sensation which passes shortly. Some people also get a feeling of needing to pass urine but this also goes away quickly.

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  • Alternatives What are the alternatives to a CT scan?

    Alternative imaging procedures include ultrasound and MRI scans. Your doctor will discuss with you which procedure is most suitable for you.

  • The procedure What happens during a CT scan?

    A CT scan usually takes about 20 minutes.

    Depending on which part of your body needs to be scanned, you may be asked to remove your clothing and put on a hospital gown. There will usually be a private area where you can do this. You may also be asked to remove any jewellery, glasses, contact lenses, dentures, hair clips and hearing aids.

    You will be asked to lie on the scanner table, which slides into or out of the CT scanner ring. Your radiographer will position the table so that the part of your body needing to be scanned is in the centre of the scanner. You may also be asked to hold your breath or not to swallow at certain points during the scan. For the rest of the time, it's important to lie very still. The CT scanner will usually make some whirring noises when you’re inside.

    Your radiographer will operate the scanner from a control room behind a window. He or she will be able to see, hear and speak to you at all times during the procedure.

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  • Aftercare What to expect afterwards

    When the scan is complete, the scanner table will move back out of the scanner ring and you will be helped down. You will usually be able to go home when you feel ready.

    Your results will be reviewed by a radiologist (a doctor who specialises in using imaging methods to diagnose medical conditions). Usually, a report will be sent out by the radiologist to your GP or the doctor who referred you for the test; your scan may also be sent. This can take several days. Before you go home, ask your radiographer when you can expect to get your results.

  • Risks What are the risks?

    As with every procedure, there are some risks associated with having a CT scan. We have not included the chance of these happening as they are specific to you and differ for every person.

    You're exposed to natural background radiation in the atmosphere all the time. Different radiology procedures expose you to different doses of radiation. For example, the amount of radiation you receive from a CT scan is more than other types of radiology procedures, such as a plain X-ray. However, doctors are trained to keep your exposure to a minimum. Ask your radiographer to explain how these risks apply to you.


    Complications are when problems occur during or after the procedure. Most people aren't affected.

    It's possible you may have an allergic reaction to the contrast medium, but this is rare. If you have any itching or difficulty breathing, tell your radiographer immediately.

    If you're pregnant

    If you're pregnant, you will usually be advised not to have a CT scan as there is a risk that the radiation could harm your unborn baby. If you're pregnant, or think you might be, let your doctor or radiographer know.

  • FAQs FAQs

    Can my child have a CT scan?


    Yes, your child can have a CT scan if his or her doctor recommends it.


    If there is a good medical reason, your child can have a CT scan in the same way that an adult does.

    In some emergency situations, your child may be given a sedative before the scan. A sedative is something that helps to produce a state of calm. It may help relieve any anxiety that your child may be feeling and help him or her to relax. It will also help your child to stay still during the scan. Your child may be given a sedative either as a tablet or an injection.

    If you have any questions about sedation or whether it’s suitable for your child, discuss this with your child’s doctor.

    Can you tell me more about how the contrast medium could affect me?


    It's possible you may have an allergic reaction to the contrast medium, but the risk is very small.


    It’s not possible to predict whether you will be allergic to the contrast medium. However, you must tell your radiographer if you have kidney problems, diabetes or any allergies.

    If your kidneys don’t function well, contrast medium may cause further damage to your kidneys. It may be necessary to carry out a blood test to check your kidney function before the CT scan. Conditions such as diabetes can affect how well your kidneys function.

    If you’re allergic to the contrast medium you may get some symptoms such as:

    • feeling sick and vomiting
    • a skin rash
    • itching
    • sneezing
    • dizziness
    • a headache
    • swelling in your mouth or throat

    If you have any of these symptoms after your CT scan you must tell your radiographer immediately.

    How does the level of radiation from a CT scan compare with an X-ray?


    The amount of radiation you get from a CT scan is more than you would get from an X-ray.


    The amount of radiation used to do a CT scan is greater than with an X-ray. This is because a CT scan uses lots of individual X-rays to build up detailed three-dimensional images. Your doctor will recommend that you have a CT scan if the benefits of having it outweigh the risks from the radiation involved. But if you're concerned, speak to your doctor or radiographer for advice.

    If a CT scan involves being exposed to radiation, will I be radioactive afterwards?


    No, you won’t be radioactive after you have had a CT scan.


    The X-rays used to make CT images are only present when you're being scanned. Once the scan is finished, there won’t be any radiation left in your body.

    There are, however, some other procedures that use radioactive materials – for example, nuclear medicine tests. These involve you being given a medicine that is radioactive. The medicine will stay in your body after the test, but will gradually go away. If you’re having a nuclear medicine test, your radiographer will explain any precautions you may need to take before and afterwards.

  • Resources Resources

    Further information


    • Information for patients having a CT scan. The Royal College of Radiologists., published December 2010
    • Provan D. Oxford handbook of clinical and laboratory investigation. 2nd ed. Oxford: Oxford University Press, 2005:694–95
    • Computed tomography (CT). US Food and Drug Administration., published 20 June 2013
    • Medical X-ray imaging. US Food and Drug Administration., published 3 July 2013
    • Computed tomography (CT) scans. PatientPlus., reviewed 20 April 2011
    • Computed tomography (CT), iodine-containing contrast medium (ICCM), plain radiography/X-rays. The Royal Australian and New Zealand College of Radiologists., published 1 May 2009
    • Birchard KR. Transthoracic needle biopsy. Semin Intervent Radiol 2011; 28(1):87–97. doi:10.1055/s-0031-1273943
    • Medical radiation FAQs. Public Health England., published 5 August 2009
    • Paediatric sedation. Medscape., published 3 June 2013
    • Tests for children's cancers. Macmillan Cancer Support., published 1 April 2012
    • Joint Formualry Commitee. British National Formulary for Children (online) London: BMJ group and Pharmaceutical Press., accessed 3 October 2013 (online version)
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